Method for determining a braking distance

ABSTRACT

A method for determining a braking distance of a vehicle, wherein a coefficient of friction (COF)actual between a brake lining and a brake disc is calculated as a function of a tensioning force Fd, a vehicle velocity V and a temperature of the frictional layer Tdisc, wherein the temperature of the frictional layer Tdisc is updated dynamically on the basis of the coefficient of friction COFactual in order to calculate a braking distance which approaches reality.

CROSS REFERENCE AND PRIORITY CLAIM

This patent application is a U.S. National Phase of International Patent Application No. PCT/EP2019/079499 filed Oct. 29, 2019, which claims priority to German Patent Application No. 10 2018 129 132.9, the disclosure of which being incorporated herein by reference in their entireties.

FIELD

Disclosed embodiments relate to a method for determining a braking distance of a vehicle in the case of a friction brake.

BACKGROUND

A braking distance is an important indicator of the braking performance of a braking system of a vehicle. Provided in standard UIC 544-1, for evaluating the braking performance for the purpose of train approval, is a method for determining a braking distance according to the method of temporal increments. This type of conventional determination of the braking distance is however based on the assumption that the coefficient of friction (COF) between a brake pad and a brake disc is constant during the entire determination.

SUMMARY

Therefore, the disclosed embodiments provide a method for increasing the accuracy in determining/estimating a braking distance of a vehicle and, associated therewith, the accuracy of assessing the braking performance of a vehicle.

BRIEF DESCRIPTION OF THE FIGURES

The method according to the disclosed embodiments will be described in relation to the figures, in which:

FIG. 1 shows a flow chart which shows the major operations of the method for determining a braking distance according to the disclosed embodiments; and

FIG. 2 shows a flow chart which shows the operations of FIG. 1 in a different illustration.

DETAILED DESCRIPTION

The assumption that the Coefficient Of Friction (COF) between a brake pad and a brake disc is constant during the entire determination does however not correspond to real conditions. Therefore, the result of the calculated/estimated braking distances will often heavily deviate from real values. In actual fact, the COF is heavily dependent on temperature. While the train is travelling, the temperature of the friction layer (the disc surface or the wheel tread) varies as a function of different braking situations, for example the value of the initial speed, the value of the braking force acting on the brake disc, or of the thermal property of the brake discs. Moreover, the temperature of the friction layer increases when the train is under a heavier load.

Disclosed embodiments determine a braking distance taking into account the momentary vehicle speed, the calculated clamping force between the brake pad and the brake disc, and. in particular, a correlation between the coefficient of friction and the surface temperature. Moreover, the disclosed embodiments simultaneously take into account the heating of discs and pads, and simultaneously the change in the coefficient of friction, because the coefficient of friction influences the profile of the vehicle speed while braking, the temperature of the friction layer can be derived from the vehicle speed, and the coefficient of friction in turn is a function of the temperature of the friction layer. This means that both heating and the frictional processes are simultaneously taken into account during the deceleration procedure, since both are functions of one another, or influence one another, respectively.

FIGS. 1 and 2 show major operations of a method according to the disclosed embodiments for determining a braking distance of a vehicle.

First, a clamping force Fb between a disc and a brake pad, a vehicle speed (initial speed) V, and an ambient temperature Tambient at the time t₀ when a braking procedure starts, are entered into a COF estimator of a braking distance estimator (S1). The vehicle speed V at S1 is in this instance the initial speed V(t₀).

Subsequently, a temperature of the friction layer Tdisc(t₀) is calculated by using a thermal function (S2), wherein the thermal function takes into account at least Fb, V(t₀) and Tambient as parameters for the determination.

Thereafter, a coefficient of friction COFactual(t₀) at the time t₀ is calculated by using a COF function (S3), wherein the thermal function takes into account at least Fb, V(t₀) and Tdisc(t₀) as parameters for the determination. The calculation of the coefficient of friction COF is performed by way of an empirical formula, for example in the form of COF_i=a+b*Fb_(i−1)+c*FbA2_(i−1)+d*v_(i−1)+e*v{circumflex over ( )}2_(i−1)+f*T_(i−1)+j*T{circumflex over ( )}2_(i−1), wherein a, b, c, d, e f, j are suitable selected parameters.

Subsequently, a partial braking distance S′₁ for a predefined temporal interval Δt from to until t₁ is calculated using a vehicle dynamics function (S4), wherein t₁ is the assumed time for the next calculation, and Δt=t₁−t₀. The temporal interval Δt can be set as a variable in the vehicle dynamics function. The vehicle dynamics function here takes into account at least Fb, V(t₀), COFactual(t₀) as parameters for the calculation. V(t₁), a vehicle speed at the time t₁ is also calculated in parallel in S4, (wherein, Rm=mean friction radius pad/disc, R=wheel radius, m=static mass (=weight), and C is a conversion factor for static mass=>dynamic mass. Example: C=1.1 corresponds to 10% rotational mass which has to be decelerated in addition to the static mass).

In operation S5, operations S2 to S4 are repeated until the vehicle speed V corresponds to the terminal speed (for example, zero), that is to say that the braking procedure ends.

A repetition in the context of S5 will be described hereunder so as to explain the principle of the calculation of S5. Next, the thermal function uses the result of the coefficient COFactual(t₀) of the last S3 and the result of the vehicle speed V(t₁) of the last S4 and recalculates the temperature of the friction layer, this resulting in Tdisc(t₁) (S2). Thereafter, the COF function uses the updated temperature of the friction layer Tdisc(t₁) and the vehicle speed V(t₁) and recalculates the coefficient of friction, this resulting in COFactual(t₁) (S3). Fb here is externally predefined (for example, by brake force distribution algorithms) In simple cases Fb=constant thus is a special case. Fb remains constant during an integration operation. Tambient is not used for Fb, but only for calculating Tdisc (in each operation).

Subsequently, the vehicle dynamic function uses the new coefficient of friction COFactual(t₁) and calculates a partial braking distance S′₂ for a temporal interval Δt from t₁ to t₂, wherein Δt=t₂−t₁. In parallel, the vehicle dynamics function also calculates V(t₂), the latter being a vehicle speed at the time t₂. Thereafter, in S5, the calculations of S2-S4 for the times t₃, t₄, t₅, . . . are repeated until the vehicle speed is zero (V=0).

At the end, all partial braking distances S′₁, S′₂, . . . are added or integrated, respectively, and as a result an overall braking distance Stotal is calculated (S6).

Disclosed embodiments take into account the dynamic coefficient of friction COF between the discs and the pads, and the temperature of the friction layer Tdisc, wherein the COF and Tdisc are calculated and updated for all individual calculation operations. A result for the braking distance which approximates reality can be calculated/estimated therefrom. The smaller the setting of the temporal interval Δt, the more accurate the result of the overall braking distance Stotal. 

1. A method for determining a braking distance, the method comprising: entering a clamping force between a brake pad and a brake disc Fb, a vehicle speed V, and an ambient temperature Tambient; calculating a temperature of a friction layer Tdisc; calculating a coefficient of friction COFactual between the brake pad and the brake disc; calculating a partial braking distance S′ of a temporal interval and updating V; repeating the calculating a temperature of a friction layer Tdisc, calculating a coefficient of friction COFactual between the brake pad and the brake disc, and calculating a partial braking distance S′ of the temporal interval and updating V until V=0, wherein in each repetition Tdisc is updated by calculating a temperature of a friction layer Tdisc; updating COFactual by calculating the coefficient of friction COFactual between the brake pad and the brake disc; calculating a new partial braking distance S′ for a next temporal interval and the vehicle speed V is updated; and integrating all S′ and determining an overall braking distance Stotal.
 2. The method of claim 1, wherein in the calculating of the partial braking distance S′ of the temporal interval and updating V, an additional braking force generated by an electro-dynamic brake, and/or a braking force generated by a magnetic rail brake, is taken into account.
 3. The method of claim 1, wherein in the calculating of the partial braking distance S′ of the temporal interval and updating V, setting of the temporal interval is enabled.
 4. The method of claim 1, wherein in calculating a temperature of a friction layer Tdisc, the temperature of a friction layer emanates from another source.
 5. A non-transitory computer program product including program code which when run on a computer processor carries out the method claim
 1. 6. A non-transitory computer program product including program code which when run on a computer processor carries out the method of claim
 2. 7. A non-transitory computer program product including program code which when run on a computer processor carries out the method of claim
 3. 8. A non-transitory computer program product including program code which when run on a computer processor carries out the method of claim
 4. 9. The method of claim 2, wherein in the calculating of the partial braking distance S′ of the temporal interval and updating V, setting of the temporal interval is enabled.
 10. The method of claim 9, wherein in calculating a temperature of a friction layer Tdisc, the temperature of a friction layer emanates from another source. 